Field of the Invention
The invention relates to a device for generating an air stream in a duplicating machine, in particular in a printing machine. The invention relates, furthermore, to methods for guiding and depositing flat products, in particular print carriers, in duplicating machines. The invention also relates to a method for the zonal powdering of print carriers in duplicating machines. The invention also relates to a delivery configuration and to a method of guiding and/or transporting flat products.
It is known, in duplicating machines, for example in printing machines, which process sheetlike or weblike flat products, in particular print carriers, to assist the transport of the flat products through the use of an air stream.
In this way, for example, in rotary printing machines paper sheets are guided by transport grippers at a front edge of the sheets and, in addition, their trailing sheet surface is supported by an air cushion. Thus a situation can be prevented where the sheet touches components of the printing machine and is damaged or where smudging of the freshly printed-on printing ink occurs.
Such a guide device for a sheet with an air flow between the sheet and a sheet guide surface is known from Published, Non-Prosecuted German Patent Application No. DE 43 08 276 A1 corresponding to U.S. Pat. No. 5,497,987. A respective sheet is exposed to an air flow generated through the use of a plurality of air jets which emerge from flow ducts provided in the sheet guide surface, the flow ducts having the form of perforations at the sheet guide surface. These perforations are provided with blowing nozzles which are connected to a compressed-air source via blowing-air lines, and, where appropriate, the blowing-air lines can be opened and closed individually or in functional groups through the use of adjustable valves.
A guide device of the type described above has the disadvantage that it is not possible to set the air flow accurately, since the strength of the individual air jets cannot be regulated in a continuous manner. Moreover, in order to modify the generated flow profile, it would be necessary to exchange the guide device for a second guide device having a modified nozzle configuration.
Further, fans may be used in deliveries of printing machines. Through the use of such fans, the printed sheets, which are released by the transport grippers, are pressed downward onto a depositing pile. German Patent No. DE 34 13 179 C2, corresponding to U.S. Pat. No. 4,643,414, describes a control and regulating device of a sheet delivery for sheet-processing machines. In this case, the sheets are supported, during depositing, by an air flow which is caused by blowing fans provided above the transport path of the sheets. The fans may be driven individually, in longitudinal or transverse rows, in a diagonal row or in any combination at a higher or lower rotational speed or else switched off completely.
However, since the fans have very large dimensions, only a small number can be provided adjacently to the sheet transport path, and therefore it is not possible to generate a desired flow field with a high degree of inaccuracy.
Furthermore, blower units are also used during the powdering of freshly printed sheets in the delivery of a printing machine. Published, Non-Prosecuted German Patent Application No. DE 197 33 691 A1, corresponding to U.S. Pat. No. 6,038,998, shows a sheet-processing rotary printing machine, in which the sheets are guided over an air cushion. In this case, above the transport path, powder nozzles are provided, through the use of which the powder is applied to the sheets in an air/powder mixture. Powder nozzles which are acted upon by powdering gas may also be provided in guide plates provided below the sheet transport path.
In the described device for the powdering of sheets, undesirable turbulences of the air/powder mixture may occur, with the result that the powder settles on components of the printing machine, so that these have to be cleaned regularly. Accurate zonal powder application is not possible with the aid of the device described.
Moreover, the prior art, such as, for example, U.S. Pat. No. 5,006,761 and U.S. Pat. No. 4,210,847, discloses fans which, instead of using a rotating propeller, utilize the phenomenon of electrical discharge. U.S. Pat. No. 4,210,847 shows a fan element having an outer cylindrical nonconductive housing, at one end of which a grounded grid is mounted, while a wire applied to a voltage is provided at the other end. When a voltage of up to 20 kV is applied, discharge takes place at the front uninsulated end of the wire, thus generating, in the vicinity of the wire, ions which are accelerated toward the grounded grid due to the prevailing electrical field. By pulse transmission, even noncharged air molecules are accelerated in the direction of the grid aperture of the fan element, with the result that an air stream of up to 500 ft./min (about 15 m/min) is obtained. Such a fan element is distinguished by a high degree of reliability, along with a simple configuration and a low weight, and by an air stream which can be controlled by the applied voltage.
U.S. Pat. No. 5,006,761 describes a similar device for generating an air stream, which, in addition to preventing point discharges and reducing the production of toxic gases due to the discharges, provides an essentially spherical body which is attached to the tip of the discharge wire, with the result that a discharge takes place uniformly on the outer surface of the discharge wire.
Moreover, a German company named ETR-GmbH in Dortmund, Germany has developed a fan which likewise utilizes the acceleration of charged air molecules between a discharge electrode and a target electrode for flow generation and which reaches flow velocities of up to 3 m/s. It is also proposed to combine individual fans into fan configurations, for example as a planar checkered configuration, in order thereby to increase the flow cross section. In the case of a cross-sectional area of 1 m2, a volume flow of 11,000 m3/h can be generated through the use of the fans described.